The present invention generally relates to composite materials and processes suitable for producing components from composite materials. More particularly, this invention relates to materials and processes by which constituents of such materials can undergo compounding and forming to produce preforms that can be cured to produce, for example, panels suitable for use in an abradable seal in the fan section of a gas turbine engine.
Gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to a combustor section of the engine where fuel is added to the air and the resulting air/fuel mixture is ignited. Afterwards, the resulting combustion gases are delivered to a turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine rotor to drive the compressor section of the engine.
Turbofan engines have a fan at the front of the engine that compresses incoming air. A portion of the compressed air is delivered to the combustor section through the compressor section, while the remainder bypasses the compressor and combustion sections and instead is delivered via a bypass duct to the rear of the engine, where the bypassed air exits through a fan exit nozzle to produce additional thrust. In high bypass turbofan engines of types widely used in large aircraft operating at subsonic speeds, including those used by commercial airlines, the fan is relatively large and a larger portion of the compressed air flows through the bypass duct to produce most of the thrust generated by the engine. Accordingly, the operation of the fan has a significant impact on the thrust and specific fuel consumption (SFC) of high bypass turbofan engines. Reductions in SFC are important to airlines for the purpose of reducing airline operating costs.
In most turbofan engines, the fan is contained by a fan case that is equipped with a shroud. The shroud circumscribes the fan and is immediately adjacent the tips of the fan blades, such that the shroud serves to channel incoming air through the fan so that most of the air entering the engine will be compressed by the fan. However, a small portion of the incoming air is able to bypass the fan blades through a radial gap present between the tips of the fan blades and the shroud. In aircraft turbofan engines and particularly high bypass turbofan engines, SFC can be significantly affected by limiting the amount of air that bypasses the fan blades through this gap.
During the normal operation of an aircraft turbofan engine, the tips of the fan blades are very likely to rub the shroud. Rubbing contact between the fan blade tips and shroud tends to increase the radial gap between the shroud and the fan blade tips, thereby reducing engine efficiency. To mitigate damage to the blade tips from rub encounters, the portion of the shroud adjacent the fan blade tips is often covered with an abradable material capable of sacrificially abrading away when rubbed by the blade tips. The abradable material is often provided in the form of arcuate panels or sectors that are mounted to the shroud to define a continuous abradable seal that circumscribes the fan blades. Common abradable materials for use in fan sections of turbofan engines contain an expandable material that, during processing to form the abradable material, is expanded to have a substantially constant cross-sectional thickness (“radial thickness”). As described in U.S. Pat. No. 5,388,959, known abradable materials include polymer-based materials, and particularly low-density syntactic foam materials that contain an epoxy resin, micro-balloons, and a reinforcement material, for example, chopped polymeric fibers.
The fan section, shroud and abradable panels are manufactured to achieve tolerances that minimize the initial radial gap between the fan blade tips and the surface of the abradable seal formed by the abradable panels. In some instances, these tolerances are intended to avoid any significant rubbing between the blade tips and abradable material. For example, minimal radial gaps may be achieved by reducing variations in the lengths of the fan blades, the radial location of the fan disk, or the fan case diameter. The inner surfaces of the abradable panels must typically be machined to achieve the diametrical dimensions required for the shroud assembly, particularly if the abradable material is of the type described above that expands during curing.
In additional to diametrical tolerances, to maintain desirable aerodynamic efficiencies associated with a small radial gap, abradable panels are often formed to achieve a desirable flowpath geometry through the creation of a surface contour that closely matches the contour of the fan blade tips. As an example, each abradable panel may be formed to have an axial profile so that when assembled within the shroud, the diameter defined by the radially inward surfaces of the panels immediately surrounding the fan blades decreases in the aft direction of the engine. However, if the abradable material is formed from an expandable foam material of a type described above, such that the abradable material initially has a substantially constant cross-sectional thickness, grinding or other suitable machining operations must be performed to produce the desired surface profile on the surfaces of the abradable panels. Inherently, this operation generates scrap material, increases raw materials cost, and increases labor costs. Another drawback of conventional expandable foam materials is that the expansion process typically creates a generally constant density throughout the abradable material, which may not be necessarily desirable for abradable panels used in the fan section of a turbofan engine.
Durability is also a consideration for abradable seals in fan sections of turbofan engines. In particular, regions of an abradable seal located upstream of the fan blades are more susceptible to direct impingement from particles entering the engine. While a low density material is typically desirable for the region of an abradable seal most likely to be rubbed by the fan blades, higher densities forward of the blades can have the desirable effect of improving erosion resistance.
In view of the above, it should be appreciated that improved performance of abradable panels for fan blade shrouds is constantly sought to improve the SFC of turbofan engines. However, an ongoing challenge is the ability to achieve such improvements with abradable panels whose geometries must be consistently producible to have relatively complex axial profiles.